The invention relates to fine-particled, aqueous polymer dispersions based on styrene/(meth)acrylate copolymers, processes for their preparation and their use as sizes for paper, cardboard and board.
The polymer dispersions according to the invention are particularly suitable as sizes for the production of graphic papers which are used for modern printing processes since they both produce a good printed image by inkjet printing and have good toner adhesion, as required, for example, for use in laser printers or copiers.
Sizes for paper which are based on styrene/acrylate dispersions are known.
Thus, Japanese Application JP 58/115196 describes aqueous dispersions based on styrene/acrylate copolymers, grafted onto water-soluble high molecular weight polyhydroxy compounds including starch, as paper strength agents having a sizing effect. These graft copolymers are obtained by polymerizing styrene and an acrylate, such as n-butyl acrylate, in the presence of an aqueous solution of starch with the formation of an aqueous dispersion. In the process described, starch is used in high molecular weight form and is not further degraded before the polymerization. The initiators used, such as potassium peroxodisulphate, ammonium peroxodisulphate or 2,2xe2x80x2-azobis(amidinopropane) dihydrochloride, moreover have unsatisfactory grafting activity, so that only coarse-particled dispersions having a low grafting yield are obtained, which dispersions can be used for increasing the strength, but whose sizing effect is unsatisfactory. In particular, the sizing effect of these products declines on papers which have been engine-sized beforehand with alkyldiketene (AKD) or alkenylsuccinic anhydride (ASA), as are usually used for the production of graphic papers, and in the case of acidic inks as are used, for example, in the Hercules sizing test for testing the sizing effect.
European Patent Application EP-A 257 412 and EP-A 276 770 claim graft copolymers of acrylonitrile and acrylates on starch, which are likewise used in the form of fine-particled aqueous dispersions for paper sizing.
Fine-particled size dispersions which are obtained by emulsion polymerization of monomers, such as, for example, acrylonitrile, butyl acrylate or styrene, in the presence of polymeric anionic emulsifiers containing sulpho groups are furthermore known (cf. EP-A 331 066 and EP-A 400 410).
These dispersions known from the prior art have excellent performance characteristics in particular on neutral and chalk-containing papers. However, they have low stability to divalent and trivalent cations, such as, for example, Ca2+ or Al3+. Under unfavourable conditions in practice, this can lead to precipitates in the size press and hence to impairment of its sizing effect.
Fine-particled polymer dispersions of starch graft copolymers based on styrene/(meth)acrylates having improved performance characteristics have now been found.
The present invention relates to aqueous dispersions obtainable by free radical emulsion copolymerization of ethylenically unsaturated monomers in the presence of starch, characterized in that
(a) 30 to 60% by weight of at least one optionally substituted styrene,
(b) 60 to 30% by weight of at least one C1-C4-alkyl (meth)acrylate,
(c) 0 to 10% by weight of other ethylenically unsaturated copolymerizable monomers
are used as ethylenically unsaturated monomers,
(d) 10 to 40% by weight of degraded starch having a molecular weight Mn=500 to 10,000 are used as starch, the sum (a)+(b)+(c)+(d) being 100%,
and a graft-linking, water-soluble redox system is used as free radical initiator for the free radical emulsion polymerization.
Suitable monomers of group (a) are styrene and substituted styrenes, such as xcex1-methylstyrene or vinyltoluene or mixtures thereof.
Suitable monomers of group (b) are C1-C4-alkyl acrylates, C1-C4-alkyl methacrylates or mixtures thereof, such as, for example, n-butyl, iso-butyl, tert-butyl or 2-butyl acrylate and the corresponding butyl methacrylates, and furthermore methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate or propyl methacrylate. A mixture of at least two isomeric butyl acrylates is preferred, it being possible for the mixing ratio to be 10:90 to 90:10. Mixtures of n-butyl acrylate and tert-butyl acrylate and mixtures of n-butyl acrylate and methyl methacrylate are particularly preferred.
Suitable monomers of the group (c) are further ethylenically unsaturated monomers, such as ethylhexyl acrylate, stearyl acrylate, stearyl methacrylate and further esters of acrylic and methacrylic acid with alcohols which have more than four C atoms, and furthermore acrylonitrile, methacrylonitrile, acrylamide, vinyl acetate or anionic comonomers, such as acrylic acid, methacrylic acid, styrenesulphonic acid. Particularly preferred monomers of group (c) are acrylic acid and styrenesulphonic acid.
The % by weight of components (a) to (d) relate to the total solids content of the dispersion, i.e. the sum of the amounts by weight of components (a) to (d).
Natural starches, such as potato, wheat, maize, rice or tapioca starch, are suitable as starch, potato starch being preferred. Starch types having a high amylopectin content of 80% or higher are preferably used. Potato starch having an amylopectin content  greater than 95% is particularly preferred.
It is also possible to use chemically modified starches, such as hydroxyethyl- or hydroxypropyl-starches, or starches which contain anionic groups, such as, for example, phosphate starch, or cationic starches which contain quaternized ammonium groups, a degree of substitution DS=0.01-0.2 being preferred. The degree of substitution DS indicates the number of cationic groups which are contained in the starch on average per glucose unit. Amphoteric starches which contain both quaternary ammonium groups and anionic groups, such as carboxylate and/or phosphate groups, and which optionally can also be chemically modified, for example hydroxyalkylated or alkyl-esterified, are particularly preferred.
The starch (d) to be used according to the invention is obtained by subjecting said starch types to oxidative, thermal, acidic or enzymatic degradation. Oxidative degradation of the starch is preferred. Oxidizing agents, such as hypochlorite, peroxodisulphate or hydrogen peroxide, or combinations thereof, which are preferably used in succession to establish the desired molecular weight of the starch, are suitable for the degradation. Starch degradation with hypochlorite, as usually carried out for improving the dissolution properties of the starch, and a further degradation, for example with hydrogen peroxide, which can be carried out, for example, shortly before the subsequent graft copolymerization, is particularly preferred. In this case, hydrogen peroxide (calculated as 100%) is used in concentrations of 0.3 to 5.0% by weight, based on starch employed. The amount of hydrogen peroxide depends on the molecular weight to which the starch is to be degraded.
The starches (d) degraded in this manner preferably have an average molecular weight Mn of 500 to 10,000, with the result that, on the one hand, good dispersing of the emulsion polymers is ensured and, on the other hand, premature crosslinking and precipitation of the polymerization batch is avoided. The average molecular weight of the degraded starch can readily be determined by gel chromatographic analysis processes after calibration, for example with dextran standards, by known methods. Viscosimetric methods, as described, for example, in xe2x80x9cMethods in Carbohydrate Chemistryxe2x80x9d; Volume IV, Academic Press New York and Frankfurt, 1964, page 127xe2x80x9d, are also suitable for the characterization. The intrinsic viscosity thus determined is preferably 0.05 to 0.12 dl/g.
The polymerization is carried out as a rule by a procedure in which both the monomers, either individually or as a mixture, and the free radical initiators suitable for initiating the polymerization are added to the aqueous solution of the degraded starch.
To increase the dispersing effect, anionic or nonionic low molecular weight emulsifiers, such as sodium alkanesulphonate, sodium dodecylsulphate, sodium dodecylbenzenesulphonate, sulphosuccinic esters, fatty alcohol polyglycol ethers, alkylaryl polyglycol ethers, etc., can be added to the polymerization batch but as a rule impair the sizing effect and generally tend to undesired foam formation. The polymerization is therefore preferably carried out in the absence of emulsifiers.
However, polymeric anionic emulsifiers which contain sulpho groups, for example based on maleic anhydride copolymers or on star oligourethanes, as described; for example, in European Patent Application EP-A 331 066 and EP-A 400 410.
The polymerization is usually carried out in the absence of oxygen, preferably in an inert gas atmosphere, for example under nitrogen. During the polymerization, thorough mixing with the aid of a suitable stirrer should be ensured.
The polymerization can be carried out both by the feed process and by a batch process at temperatures between 30 and 100xc2x0 C., preferably between 70 and 95xc2x0 C. Temperatures above 100xc2x0 C. are possible if a pressure reactor under superatmospheric pressure is employed. A continuous polymerization in a stirred kettle cascade or a flow tube is also possible.
In the feed process, which is preferred for obtaining a fine-particled dispersion, the monomers and the free radical initiator are metered uniformly into the starch solution in a stirred kettle. To achieve particular effects, nonuniform or staggered addition of individual components may also be effected. The reaction times are between 0.5 and 10 hours, preferably between 0.75 and 4 hours.
Graft-linking water-soluble redox systems are suitable for initiating the poly-merization. Conventional water-soluble initiators, such as potassium peroxo-disulphate, sodium peroxodisulphate, ammonium peroxodisulphate, hydrogen peroxide, etc., optionally in the presence of conventional reducing agents, such as sodium sulphite, sodium disulphite, sodium bisulphite, sodium dithionite, ascorbic acid and the sodium salt of hydroxymethanesulphinic acid, etc., are suitable for the polymerization but lead to coarse-particled dispersions which have only an inadequate degree of grafting and are unsatisfactory in their sizing effect. Furthermore, oil-soluble organic peroxides or azo initiators which are only slightly water-soluble are less suitable as free radical initiators since they give only unstable dispersions containing large amounts of coagulum, which are unusable for the desired purpose.
Suitable water-soluble initiator systems having high grafting activity are redox systems comprising hydrogen peroxide and heavy metal ions such as cerium, manganese or iron(II) salts, as described, for example, in Houben-Weyl xe2x80x9cMethoden der organischen Chemie, [Methods of Organic Chemistry], 4th Edition, Volume E20, page 2168xe2x80x9d. The redox system comprising hydrogen peroxide and an iron(II) salt, such as iron(II) sulphate, which gives fine-particled dispersions having a high grafting yield, is particularly suitable. The grafting yield is understood as meaning the proportion of the polymer which is chemically coupled to the starch after the end of polymerization. The grafting yield should be as high as possible in order to achieve fine-particled and effective dispersions.
The polymerization is usually carried out in such a way that the heavy metal salt of the redox system, such as, for example, the iron(II) salt, is added to the batch before the polymerization, while hydrogen peroxide is metered in simultaneously with the monomers but separately. Iron(II) salt is usually used in concentrations of 10-200 mg/l Fe++ ion, based on the total dispersion, higher or lower concentrations also being possible. Hydrogen peroxide (calculated as 100%) is added in concentrations of 0.2 to 2.0% by weight, based on monomer. This amount should be added to the amount of hydrogen peroxide which is used for the starch degradation.
In addition, the above-mentioned conventional initiators can be concomitantly used. The addition of further reducing agents, which are preferably initially introduced with the iron salt before the polymerization, has particular advantages. Suitable reducing agents are, for example, sodium sulphite, sodium disulphite, sodium bisulphite, sodium dithionite, ascorbic acid and the sodium salt of hydroxymethane-sulphinic acid.
The molecular weight of the grafted-on polymer can additionally be adjusted by the concomitant use of chain-transfer agents or regulators, such as, for example, n-dodecyl mercaptan, tert-dodecyl mercaptan, n-butyl mercaptan, tert-butyl mercaptan, etc.
Polymerization with the redox system comprising hydrogen peroxide and heavy metal ions give fine-particled dispersions having a good sizing effect. However, the polymerization generally seizes at conversions of, for example, 95 to 98%, based on monomer used, so that relatively high residual monomer contents remain, necessitating complicated monomer removal, for example by distillation and devolatilization in vacuo.
It was surprisingly found that the polymerization can be continued to very high conversion and to very low residual monomer content if an oil-soluble, sparingly water-soluble free radical initiator is added for subsequent activation after the polymerization with the water-soluble redox system, and the polymerization is completed therewith.
Suitable oil-soluble, sparingly water-soluble free radical initiators are, for example, customary organic peroxides, such as dibenzoyl peroxide, di-tert-butyl peroxide, tert-butyl hydroperoxide, cumyl hydroperoxide or bis-cyclohexyl peroxydicarbonate.
In this case, polymerization is first carried out, for example, with hydrogen peroxide and iron(l) sulphate, with high grafting yield to a conversion of about 95 to 98%, based on monomer used, and, for example, an oil-soluble, sparingly water-soluble organic peroxide is then added for subsequent activation, it being possible to achieve a conversion  greater than 99.8% and a residual monomer content  less than 100 ppm and to dispense with monomer removal.
Here, sparingly water-soluble is intended to mean that less than 1% of the organic peroxide is completely soluble in water at room temperature.
In polymerization processes without subsequent activation, the residual monomer content is so high that subsequent monomer removal, for example by steam distillation or by passing in a gas stream, is required in order to keep the residual monomer content below required limits and to avoid odour annoyance during use. The grafting reaction with a water-soluble redox system, such as, for example, hydrogen peroxide and iron(II) sulphate, and the subsequent activation with a sparingly water-soluble organic peroxide, such as tert-butyl hydroperoxide, is therefore particularly preferred. Without adversely affecting the quality of the dispersion, it is thus possible to obtain residual monomer contents  less than 100 mg/kg, so that complicated monomer removal can be dispensed with.
The polymerization is carried out at pH values of 2.5 to 9, preferably in the weakly acidic range at pH values of 3 to 5.5. The pH value can be adjusted to the desired value before or during the polymerization using customary acids, such as hydrochloric acid, sulphuric acid or acetic acid, or using bases, such as sodium hydroxide solution, potassium hydroxide solution, ammonia, ammonium carbonate, etc. Adjustment of the pH value to 5 to 7 after the polymerization with sodium hydroxide solution, potassium hydroxide solution or ammonia is preferred.
The concentration of the dispersions according to the invention is between 10 and 40% by weight, preferably between 18 and 30% by weight. The viscosity of a 25% strength dispersion is between 3 and 30 mPas.
The dispersions according to the invention have a very small particle size of less than 100 nm, preferably 50 to 90 nm. The particle size can be determined, for example, by laser correlation spectroscopy or by turbidity measurement. In the turbidity measurement of the polymer dispersions according to the invention, the latter, on dilution to an active ingredient content of 2.5% (1:10), have an absorbance between 0.25 and 1.2, measured in a 1 cm cell at 660 nm.
To increase the shelf-life, it is advantageous to bind the heavy metal ions used in the redox system subsequently to the polymerization by complexing agents, for which purpose complexing agents such as ethylenediaminetetraacetic acid, nitrilotriacetic acid, diethylenetriaminepentaacetic acid, polyaspartic acid, iminodisuccinic acid, citric acid or their salts are suitable. The amount of complexing agents used depends on the amount of heavy metal salt used. Usually, the complexing agents are used in concentrations of 1 to 10 mol, preferably in concentrations of 1.1 to 5 mol, per mol of heavy metal ion.
The polymer dispersions according to the invention are surface sizes having weakly anionic, amphoteric or cationic charge character and little tendency to foam formation, which have a broad application spectrum. They are suitable for surface sizing of all paper qualities produced in practice, for example of raw papers which are alum-containing, alum-free, filled with kaolin or chalk and containing groundwood or waste paper and which can be produced under acidic and under neutral or alkaline conditions and which may be unsized or may be presized in the paper pulp, for example with alkylketene dimer or alkenylsuccinic anhydride. Particularly those polymer dispersions according to the invention which contain a mixture of at least two isomeric (meth)acrylic acid (C1-C4) esters as monomer components are distinguished by an outstanding sizing effect on papers engine-sized beforehand and a substantially improved sizing effect with respect to acidic inks.
The dispersions according to the invention can be processed by all methods customary in surface sizing and can be applied to the surface of paper in the size press liquor. Use in aqueous solution together with 5 to 20% by weight of starch and optionally pigments and optical brighteners in the size press or in modern application units, such as a film press, speedsizer or gate-roll, is customary. The amount of size in the liquor depends on the desired degree of sizing of the papers to be finished. Usually, the concentration of the dispersions according to the invention in the liquor is between 0.1 and 2.0% by weight of solid substance, preferably between 0.2 and 1.0% by weight. The amount applied to the paper is determined by the liquid absorption of the optionally presized papers. The liquid absorption is to be understood as meaning the amount of size press liquor which, based on the dry fibre, can be absorbed by the latter and which can be influenced, inter alia, by the presizing in the paper pulp. Depending on the liquid absorption, the amount of size absorbed by the paper is 0.03 to 1.2% by weight of solid substance, based on dry fibre, preferably between 0.1 and 0.8% by weight.
In addition, the size press liquors may contain fine-particled pigments for improving the printability, such as, for example, chalk, precipitated calcium carbonate, kaolin, titanium dioxide, barium sulphate or gypsum. Furthermore, the addition of optical brighteners for increasing the whiteness, optionally with the addition of carriers, such as, for example, polyethylene glycol, polyvinyl alcohol or polyvinylpyrrolidone, is customary in the case of use on graphic papers. The good compatibility of the dispersions according to the invention with optical brighteners is particularly advantageous, so that papers having high whiteness can be obtained. Surprisingly, dispersions having amphoteric and cationic charge character can also be used together with optical brighteners without precipitates occurring or a decrease in the whiteness being observed, in contrast to customary cationic sizes.
Also particularly advantageous is the insensitivity of the dispersions according to the invention to the addition of electrolytes, such as Na, Ca or Al ions, which may in many cases be contained in the size press liquor, for example through migration from the raw paper to be processed, or are deliberately added for increasing the conductivity.
The size dispersions according to the invention are particularly suitable for the production of graphic papers which are used for all customary modern printing processes. In inkjet printing, for example, high ink adsorptivity and rapid drying without strike-through are required in combination with good ink hold-out, production of a high ink density and high resolution and good smudge and water resistance. In colour printing, crisp edges are required, and the individual coloured inks must not run into one another and should have high colour intensity, brilliance and lightfastness. These requirements can be met in an outstanding manner by the dispersions according to the invention. Dispersions having amphoteric or cationic charge character have particular advantages in the water fastness of the inkjet print through better fixing of the inkjet dye.
For using the papers treated with the dispersions according to the invention in electrophotographic printing processes, such as in laser printers or copiers, good toner adhesion is simultaneously required, i.e. the toner must adhere to the paper with high smudge resistance. This requirement, too, can be met in an outstanding manner by using the dispersions according to the invention on papers presized, for example, with alkyldiketene.